Selection of transmitter responses at sites of neurite contact during synapse formation between identified leech neurons.

1. Pressure sensitive (P) neurons of the leech Hirudo medicinalis show both an inhibitory, Cl(‐)‐dependent response and a depolarizing, cationic response to pipette application of serotonin (5‐HT). Serotonergic Retzius (R) neurons in culture reform inhibitory, Cl(‐)‐dependent synapses with P neurons but fail to elicit the extrasynaptic, depolarizing response to 5‐HT. We have examined the localization of the selection of 5‐HT responses by testing the sensitivity of P cell growth cones and neurites to 5‐HT application. 2. As measured by intracellular recording at the P cell soma, synaptic release of 5‐HT from R cell processes activated only the Cl(‐)‐dependent response in P cell neurites. Focal application of 5‐HT from a micropipette depolarized uncontacted P cell growth cones and neurites. In contrast, processes from the same P cells that were contacted by R cells were rarely depolarized by 5‐HT application unless the application pipette was moved along the neurites away from the sites of contact. 3. The channels underlying the depolarizing response to 5‐HT were identified in patch clamp recordings from P cell growth cones. These cation channels showed rare, brief openings in the absence of 5‐HT. Application of 5‐HT in the bath (outside the patch pipette) increased channel activity in uncontacted P cell growth cones but not in growth cones of the same P cells contacted by R cells. 4. We conclude that the selection of transmitter responses during synapse formation was localized to discrete sites of contact between the synaptic partners.